1. Field of the Invention
This invention relates in general to spread spectrum receivers and in particular to GPS navigation systems such as those used in terrestrial navigation for cars, trucks and other land vehicles.
2. Description of the Prior Art
Car navigation is conventionally performed using highway and street maps aided, to some degree, by distance measurements from external sensors such as odometers. Improvements over the last 10 years in Global Positioning System, or GPS, satellite navigation receivers has spawned several GPS car navigation systems.
Conventional GPS car navigation systems use the last known position of the vehicle, and the destination data, to compute a route data base, including route and turning data derived from a pre-existing map data base. GPS receivers are conventionally operated with a minimum of 3 or 4 satellites distributed across the visible sky in order to determine, or at least estimate, the four necessary unknowns including x.sub.user, y.sub.user and z.sub.user which provide three orthogonal coordinates to locate the user as well as t.sub.user which provides the required satellite time. Techniques such as time or clock hold and altitude hold, in which the unknown time or altitude is assumed to remain predictable from a previously determined value, e.g. z.sub.est and/or t.sub.est, have permitted operation of GPS receivers with less than 4 satellites in view. In particular, terrestrial GPS receivers have been operated with as few as 2 satellites to provide a 2 dimensional position solution using both clock and altitude hold.
Because continuous reception from 4 GPS satellites is often difficult to maintain in a car navigation environment, and known clock and altitude hold techniques can only permit operation with at least 2 satellites, known conventional car navigation systems have typically augmented the GPS position information with information from external sensors to provide dead reckoning information. The dead reckoning information is often provided by an inertial navigation system such as a gyroscope.
Augmenting GPS data with inertial navigation data has permitted the use of GPS car navigation even when less than 4 satellites are visible, such as in tunnels and in urban situations between tall buildings. However, the resultant increased complexity and costs for such combined systems have limited their acceptance.
Conventional GPS receivers use separate tracking channels for each satellite being tracked. Each tracking channel may be configured from separate hardware components, or by time division multiplexing of the hardware of a single tracking channel, for use with a plurality of satellites. In each tracking channel, the received signals are separately doppler shifted to compensate for the relative motion of each satellite and then correlated with a locally generated, satellite specific code.
During a mode conventionally called satellite signal acquisition, delayed versions of the locally generated code for the satellite being acquired are correlated with the doppler rotated received signals to synchronize the locally generated code with the code, as received for that satellite, by determining which delay most accurately correlates with the code being received. Once synchronization has been achieved for a particular satellite, that satellite channel progresses to a tracking mode in which the doppler rotated, received signal is continuously correlated with the locally generated code for that satellite to determine position information including pseudorange information. During tracking, conventional receivers also correlate the doppler shifted received signal with one or more versions of the locally generated code at different relative delays, such as one half C/A code chip width early and late relative to the synchronized or prompt version of the code. These early and late correlations are used to accurately maintain the synchronization of prompt correlation.
When, after tracking has begun for a particular satellite, the satellite signal has been lost so that the required timing of the locally generated code for synchronization is no longer accurately known, conventional receivers reenter the acquisition mode, or a limited version of this mode, to reacquire the satellite signals by multiple correlations to resynchronize the locally generated code with the code as received. Once the locally generated code has been resynchronized with the signals as received, position information data is again derived from the signals from that satellite.
What is needed is an improved spread spectrum receiver, for example, for use in a GPS system for terrestrial navigation which does not require the complexity or costs of conventional systems. In particular, a GPS receiver is needed which can make optimal use of signals available in difficult environments, such as urban environments, with less than 4 satellites continuously in view and frequent obscuration of signals from some of the satellites.